A common technique of making spring mattresses is the so-called pocket technique. This means that the springs are enclosed in pockets, that is they are individually enclosed by a casing material. In this way, the springs will be relatively individually resilient so that they can flex individually without affecting the neighboring springs and, thus, the comfort to the user increases since his weight will be distributed more uniformly over the surface that receives the load.
A drawback of this type of mattress is, however, that it is difficult to provide thin mattresses. If the length of the springs is reduced without a corresponding reduction of the width, the spring will, especially when the length approaches the length of the diameter of the spring, have a tendency to turn in the casing, which dramatically deteriorates the comfort of the mattress. When such mattresses are to be manufactured, the existing technique requires the use of a much larger number of springs. Consequently the manufacturing will be considerably more expensive and more complicated. Besides it is difficult to prevent such mattresses from also being stiffer since too thin spring wires cannot be used.
For these reasons, it has not been possible to use spring mattresses for many purposes where thinner mattresses are required, such as for overlay mattresses, seat cushions and the like. In spite of this, spring mattresses have several properties making it desirable to use them also in these contexts, such as excellent comfort, individual flexibility, a long life and easy and inexpensive manufacture.
A solution to this problem has been presented by the same applicant in WO 00/58203. In this solution, the end portions of the casings are moved towards each other, which gives a bias of the springs, and a very compact and useful mattress. A drawback of this mattress is, however, that it is relatively complicated and expensive to manufacture. Moreover the moving together of the end portions counteracts the normally desired individual and independent resilience of the individual enclosed springs.